1. Field of the Invention
The present invention relates generally to an electronically controlled, high resolution light source, and more particularly, to a thin film electroluminescent (TFEL) edge emitter module employing a hermetically-sealed and refractive index-matched solid covering over the light-emitting face.
2. Description of the Prior Art
Electroluminescence is a phenomena which occurs in certain materials from the passage of an electric current through the material. The electric current excites the electrons of the dopant in the light emitting material to higher energy levels. Emission of radiation thereafter occurs as the electrons emit or give up the excitation energy and fall back to lower energy levels. Such electrons can only have certain discrete energies. Therefore, the excitation energy is emitted or radiated at specific wavelengths depending on the particular material.
TFEL devices that employ the electroluminescence phenomena have been devised in the prior art. It is well known to utilize a TFEL device to provide an electronically controlled, high resolution light source. One arrangement which utilizes the TFEL device to provide the light source is a flat panel display system, such as disclosed in U.S. Pat. No. 4,110,664 to Asars et al and U.S. Pat. No. 4,006,383 to Luo et al, assigned to the assignee of the present invention. In a TFEL flat panel display system, light emissions are produced substantially normal to a face of the device and so provide the light source at the device face. Another arrangement utilizing the TFEL device to provide the light source is a line array, or edge, emitter, such as disclosed in a U.S. Pat. No. 4,535,341 to Kun et al, also assigned to the assignee of the present invention. In a TFEL edge emitter system, light emissions are produced substantially normal to an edge of the TFEL device and so provide the light source at the device edge. Edge emissions by the TFEL edge emitter system are typically 30 to 40 times brighter than the face emissions by the TFEL flat panel display system under approximately the same excitation conditions.
From the above discussion, it can be appreciated that the TFEL edge emitter system of the Kun et al patent potentially provides a high resolution light source promising orders of magnitude of improved performance over the TFEL flat panel face emitter system in terms of light emission brightness. For the TFEL edge emitter device to be able to reach its full commercial potential, it must be capable of use in applications where potentially harmful contaminants, such as moisture and airborne particulates, will be present.
One packaging assembly has been devised to provide a contaminant-free environment for the TFEL edge emitter device to permit its use in such applications. Such packaging assembly is disclosed in the fourth patent application cross-referenced above. This packaging assembly includes a sealed enclosure having an internal sealed cavity surrounding the light emitting edge of the TFEL edge emitter device and a front translucent glass window through which can pass light energy emitted by the TFEL edge emitter device. Also, the packaging assembly includes an oil-like liquid which fills the internal sealed cavity. The liquid has an index of refraction which matches the index of refraction of either the front glass window or the electroluminescent (EL) stack of the TFEL edge emitter device.
One major problem which has been encountered with the packaging assembly of the above-described construction is that the sealed enclosure is so rigid that thermal expansion of the liquid inside the sealed cavity due to as little as a 20.degree. C. increase in temperature can cause the enclosure to rupture and leak. One proposal to solve this problem is disclosed in the eighth patent application cross-referenced above. In this proposal, a capacity varying mechanism, such as a rigid hollow tube having a sliding piston, a flexible sealed tube containing a gas, or a flexible bladder or diaphragm mounted across a passage to the enclosure cavity, is incorporated in the sealed enclosure. The capacity varying mechanism will accommodate thermal expansion of the liquid by increasing the enclosure cavity capacity, thereby absorbing the increase in liquid volume and preventing the rupture of the sealed enclosure. The capacity varying mechanism also will contract the capacity of the cavity as the liquid cools so as to maintain the cavity liquid-holding capacity substantially equivalent to the enclosed volume of liquid.
While these embodiments of the capacity varying mechanism have been shown to function, there a need still remains to develop a more cost-effective and efficient technique for preventing enclosure rupture and protecting the edge emitter device from contamination.